1.Field of the Invention
This invention relates to a rotor with salient poles for electric rotating machines such as a hydraulic turbine generator, and more in particular to a construction of the shielding members for sealing the gaps between adjacent poles.
2. Description of the Prior Art
In recent years, the capacity and speed of hydraulic turbine generators have been remarkably increased. This trend is especially notable with high-head turbine generators used in such as pumping-up power stations. In such a hydraulic turbine generator of large capacity and high speed, an increased peripheral speed of the rotor causes the wind friction loss to be increased in proportion to the cube of the peripheral speed. As a result, the loss of the generator is increased thereby lowering its efficiency.
An attempt to reduce the wind frictional loss has been made by forming the rotor into a cylindrical appearance. An example of such a cylindrical rotor is disclosed in U.S. Pat. No. 3,157,806 issued Nov. 17, 1964 to Eugen Wiedemann, and Japanese Utility Model Application No. 131642/74 published without examination as Laid-Open No. 58802/76.
In those rotors, as shown in FIGS. 1 and 2, a plurality of salient poles 4 each including a field core 2 and a field coil 3 wound thereon are mounted along the outer periphery of a rotor rim 1 fixed to a rotor shaft (not shown), and a nonmagnetic shielding member 5 in flat form is suspended between the upper parts of adjacent field cores 2, so that the rotor is provided with a cylindrical appearance, thereby reducing the wind frictional loss. The shielding member 5 is mounted on the field cores 2 in such a manner that a slot 6 in parallel to the rotor shaft is formed in the side 2H of the upper part of each of the adjacent field cores and a lateral edge of the shielding member 5 is fitted into the slot 6 formed to one of the field cores, while the other lateral edge of the shielding member 5 is fitted into the slot 6 formed to the other of the adjacent field cores.
In this construction, as the capacity and speed of the hydraulic turbine generator increase, the outer diameter of the rotor increases accordingly, which in turn, as well as its increased speed, increase the centrifugal force exerted on the shielding member 5, so that the thickness t of the shielding member 5 is required to be increased in order to endure the increased centrifugal force. The shielding member 5 may be constructed of a single flat plate or a plurality of flat plates disposed side-by-side. In actual cases, such a plurality of flat plates each having an appropriate width L are arranged to make up the shielding member 5 in consideration of the convenience in production of the flat plates and their fitting into the slot 6.
In such construction, however, it may occur that due to errors in production of the flat plates and/or in forming the slot 6 in the side 2H of the upper part of the field core, as well as the temperature rise during operation, the end walls of the flat plates making up the shielding member 5 fail to come into close contact with the outer periphery side wall 6S of the slot 6 but partial contact with the wall 6a, as shown in FIG. 3. As a result, all the centrifugal force exerted on the flat plates of which the weight is increased by an increased thickness t is applied to the partial contact points A and B, thus producing an excess stress locally in the wall of the slot 6. In view of the fact that the slot 6 is formed in the field core 2 at a portion 2H near to its outer periphery, as shown in FIG. 1, the height H of the top marginal portion 2H must be more than sufficient to prevent the slot 6 from being damaged by the local excess stress. This results in an increased size of the salient poles 4, thus increasing the outer diameters and weight of the rotor and stator (not shown) disadvantageously.
To point A, in particular, the centrifugal forces of two adjacent flat plates are additively imparted as shown in FIG. 3, and therefore it is subjected to a greater stress than the partial contact point B, thereby leading to an uneven stress distribution over the entire length of the wall of the slot 6.